The present invention pertains to a screw type fastener utilizable in substrates including lumber, LVL, particle board, wafer board, plastic lumber, cement board, plaster board and similar penetrable products. The screw has a substantially improved pull-out resistance in consequence of the unique thread design.
Self-tapping screws, such that the screws do not require a pilot hole, are typically employed in relation to penetrable substrates such as wood. Typically, the screws have a driving head, a threaded shank and a form of point at the tip of the screw to aid in penetration. While screws having a variety of apertured or notched threads are known for improved thread cutting during penetration, traditional threads have a thread profile which is substantially symmetrical about a normal from the shank access.
A major problem within the housing industry, and in particular wood framed housing, arises from wind uplift during high winds such as generated in hurricanes, typhoons, etc. Wind uplift applies pressure on sheathing materials which may be held to structural lumber by nails or screws. Various nail designs have been developed to improve holding capacity against wind uplift (i.e. against pull-out forces). While it is recognized that screws of a traditional design may well have equal or greater resistance to pull-out during uplift conditions than do nails, pull-out remains a problem.
One improvement in screw design and technology has been the recognition that a steeply sloping upper thread surface, such as employed in traditional wood screws, can permit substrate fibres to slide relative to the sloped thread surface during periods of intense axial tension, such as occurs in wind uplift on roofs and structures. Pull-out is facilitated by increased slope. One solution proposed is to reduce the slope on the upper thread face until almost “flat”, or normal to the axis of the screw. Such a screw is disclosed in applicants' pending application, PCT/CA2010/000747, where the slope is minimized to within 0° to 3°